1. Field of the Invention
This invention relates generally to plug assemblies and more particularly, to inflatable plug assemblies for boreholes.
2. Description of Related Art
Boreholes are drilled for the purpose of delivering an explosive charge to a selected location in, for example, an underground earth structure. The setting of the explosive charge often requires the borehole to be blocked so that the charge can be set at a desired level. Boreholes often tend to at least partially fill with water and the presence of the water can prevent successful detonation of the explosive. In attempts to overcome this problem, water proof explosives have been used. However such explosives are expensive in comparison to other explosives.
Furthermore, it is often the case that more than one explosive charge needs to be used with the explosives being positioned at spaced intervals along the borehole. This requires that the borehole plugs or other decking be placed at the selected depths along the borehole. The types of decking which have been used in the past include the application of a column of concrete or the like to a section of the borehole to space the explosive charges. Another system employs wooden, concrete or other types of discs with similar cross-sectional dimensions to the borehole which are lowered into the borehole so that they float on the surface of water within the borehole. It has also been proposed to use flexible polyester resins which float on the water and subsequently solidify. A further device which has been used comprises an inflatable plug with a time delay mechanism to allow inflation of the plug at a selected depth to thereby plug the borehole. More recently, a gel or slurry explosive has been used to replace the typical decking. Each of the above techniques suffers from certain disadvantages. Timber or concrete decking are cumbersome and generally not sufficiently reliable. Polyester resins are difficult to handle and slow to place. Internally inflated borehole plugs of the type described in U.S. Pat. No. 5,346,005, wherein a time delay mechanism permits the inflating plug to be lowered before inflation occurs may experience freezing of the time delay liquid and do not offer a ready delay time option.